WASHINGTON (AP) — Tensions rising by the day, the Obama administration said Friday it is warning Iran through public and private channels against any action that threatens the flow of oil from the Persian Gulf. The Navy revealed that two U.S. ships in and near the Gulf were harassed by Iranian speedboats last week.

Spokesmen were vague on what the United States would do about Iran’s threat to block the strategic Strait of Hormuz, but military officials have been clear that the U.S. is readying for a possible naval clash.

That prospect is the latest flashpoint with Iran, and one of the most serious. Although it currently overshadows the threat of war over Iran’s disputed nuclear program, perhaps beginning with an Israeli military strike on Iran’s nuclear structure, both simmering crises raise the possibility of a shooting war this year.

“We have to make sure we are ready for any situation and have all options on the table,” Defense Secretary Leon Panetta said, addressing a soldier’s question Thursday about the overall risk of war with Iran.

Navy officials said that in separate incidents Jan. 6, three Iranian speedboats — each armed with a mounted gun — briefly chased after a U.S. Navy ship just outside the Gulf near the Strait of Hormuz and a U.S. Coast Guard cutter in the northern Gulf. No shots were fired and the speedboats backed off.

For several reasons, the risk of open conflict with Tehran appears higher in this election year than at any point since President Barack Obama took office with a pledge to try to bridge 30 years of enmity. A clash would represent a failure of U.S. policy on several fronts and vault now-dormant national security concerns into the presidential election contest.

The U.S. still hopes that international pressure will persuade Iran to back down on its disputed nuclear program, but the Islamic regime shows no sign it would willingly give up a project has become a point of national pride. A nuclear bomb, or the ability to quickly make one, could also be worth much more to Iran as a bargaining chip down the road.

Time is short, with Iran making several leaps toward the ability to manufacture a nuclear weapon if it chooses to do so. Iran claims its nuclear development is intended for the peaceful production of energy. Meanwhile, several longstanding assumptions about U.S. influence and the value of a targeted strike to stymie Iran’s progress toward a nuclear weapon have changed. For one, the White House is no longer confident it could prevail on Israel not to launch such a strike.

An escalating covert campaign of sabotage and targeted assassinations highlighted by this week’s killing of an Iranian nuclear scientist may not be enough to head off a larger shooting war and could prod Iran to strike first.

The brazen killing of a young scientist by motorcycle-riding bombers is seen as almost surely the work of Israel, according to U.S. and other officials speaking on condition of anonymity to discuss intelligence matters. The killing on a Tehran street followed the deaths of several other Iranians involved in the nuclear program, a mysterious explosion at an Iranian nuclear site that may have been sabotage and the apparent targeting of the program with an efficient computer virus.

Iranian officials accuse both Israel and the U.S. of carrying out the assassination as part of a secret operation to stop Iran’s nuclear program. The killing came a day after Israeli military chief Lt. Gen. Benny Gantz was quoted as telling a parliamentary panel that 2012 would be a “critical year” for Iran — in part because of “things that happen to it unnaturally.”

Secretary of State Hillary Rodham Clinton and Panetta made a point of publicly denying any U.S. involvement, but the administration tied itself in knots this week over how far to go in condemning an action that could further the U.S. goal of stalling Iranian nuclear progress.

The U.S. position remains that a military strike on Iran’s known nuclear facilities is undesirable because it would have unintended consequences and would probably only stall, not end, the Iranian nuclear drive. That has been the consensus view among military leaders and policy makers for roughly five years, spanning a Republican and Democratic administration.

But during that time Iran has gotten ever closer to a potential bomb, Israel has gotten more brazen in its threats to stop an Iranian bomb by nearly any means, and the U.S. administration’s influence over Israel has declined.

Israel considers Iran its mortal enemy and takes seriously the Iranian threat to wipe the Jewish state from the map. The United States is Israel’s strongest ally and international defender, but the allies differ over how imminent the Iranian threat has become and how to stop it.

The strained relationship between Obama and Israeli Prime Minister Benjamin Netanyahu plays a role, as does the rise in influence of conservative political parties in Israel. U.S. officials have concluded that Israel will go its own way on Iran, despite U.S. objections, and may not give the U.S. much notice if it decides to launch a strike, U.S. and other officials said. The officials spoke on condition of anonymity to discuss sensitive diplomacy.

The Obama administration is concerned that Iran’s claim this week that it is expanding nuclear operations with more advanced equipment may push Israel closer to a strike.

Obama last month approved new sanctions against Iran that would target its central bank and its ability to sell petroleum abroad. The U.S. has delayed implementing the sanctions for at least six months, worried about sending the price of oil higher at a time when the global economy is struggling.

A senior commander of the Revolutionary Guard force was recently quoted as saying Tehran’s leadership has decided to order the closure of the Strait of Hormuz if the country’s petroleum exports are blocked due to sanctions.

Panetta linked the two crises Thursday, saying an Iranian nuclear weapon is one “red line” the U.S. will not allow Iran to cross and a closure of the strait is another.

“We must keep all capabilities ready in the event those lines are crossed,” Panetta told soldiers at Fort Bliss, Texas.

He did not elaborate, but the nation’s top military officer, Joint Chiefs of Staff Chairman Martin Dempsey, has said the U.S. would take action to reopen the strategic waterway. That could only mean military action, and there are U.S. warships stationed nearby.

“The United States and the international community have a strong interest in the free flow of commerce and freedom of navigation in all national waterways,” White House press secretary Jay Carney said Friday, adding that Iran is well aware of that position. “Our views are clear, we’re expressing them publicly and privately, and I’ll leave it at that.”

International talks to barter Iran out of building a nuclear weapon are nearly collapsed, the United States and several partners are on the verge of applying the toughest sanctions yet on Iran’s lifeblood oil sector, an increasingly cornered Iranian leadership is lashing out in unpredictable ways and faces additional internal pressures with a parliamentary election approaching.

All that adds up to a new equation, U.S. and Western diplomats said. A unilateral U.S. military strike on Iran’s nuclear infrastructure remains unlikely but no longer unthinkable, while the likelihood of an Israeli military strike has increased.

Immediate consequences would probably include an unpredictable spike in oil prices, ripple effects in troubled European economies and a setback for the fragile U.S. economic recovery. Longer term, a strike or a full-on war would almost surely ignite anti-American sentiment in the Middle East and beyond and empower hardline political movements in newly democratic Egypt and elsewhere.

Although the Obama administration wants to avoid conflict, it is locked in a cycle of provocation and reaction that feeds Iranian fears and may make war more likely, said Suzanne Maloney, a former State Department Iran expert now at the Brookings Institution.

“The tactics the administration has been taking means conflict becomes more likely because of the potential for miscalculation and the level of tensions and frustrations on both sides,” she said.